The invention relates to a process which is used to produce krypton and/or xenon by low-temperature fractionation of air.
The basic principles of the low-temperature fractionation of air in general and the structure of rectification systems for nitrogen-oxygen separation specifically are described in the monograph xe2x80x9cTieftemperaturtechnikxe2x80x9d [Cryogenic Engineering] by Hausen/Linde (2nd Edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). The high-pressure column is operated under a higher pressure than the low-pressure column; the two columns are preferably in heat-exchanging relationship with one another, for example via a main condenser, in which top gas from the high-pressure column is liquefied against evaporating bottom liquid from the low-pressure column. The rectification system of the invention may be designed as a conventional double column system, but may also be designed as a three-column or multicolumn system. In addition to the columns for nitrogen-oxygen separation, there may also be further apparatus for producing other air components, in particular noble gases, for example an argon production apparatus.
A process for producing krypton and/or xenon by low-temperature fractionation of air and a corresponding apparatus are known from DE 10000017 A1. In this process, a krypton- and xenon-containing fraction, specifically the bottom liquid, from the high-pressure column of the double column for nitrogen-oxygen separation is passed, without any measures which change the concentrations, into a further column which is used to produce krypton-xenon.
DE 2605305 A shows a process and an apparatus for producing krypton and/or xenon by low-temperature fractionation of air of the type described in the introduction. In this document, the first condenser-evaporator is heated by condensing top gas from a crude argon column and, at the same time, forms the bottom heating of the krypton-xenon enrichment column. All the vapour which rises in the krypton-xenon enrichment column is produced in the first condenser-evaporator.
An object of the invention is to further improve the production of krypton and xenon, and in particular to carry out this production in a particularly economic way.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects are achieved by introducing a liquid from the lower region of the krypton-xenon enrichment column into a second condenser-evaporator, which is separate from the first condenser-evaporator.
In the invention, therefore, there is a separate heat exchanger, the xe2x80x9csecond condenser-evaporatorxe2x80x9d, in which rising vapour for the krypton-xenon enrichment column is produced independently of the first condenser-evaporator, and in this way relatively low-volatility constituents are concentrated further. The second condenser-evaporator is preferably designed for bottom heating of the krypton-xenon enrichment column. It may be arranged inside this column or in a separate vessel.
The second condenser-evaporator leads to a less high oxygen concentration being established in the first condenser-evaporator, so that, on account of the correspondingly reduced temperature difference, the overall size of the first condenser-evaporator can be reduced. Moreover, there is less intensive concentration of relatively low-volatility constituents in the first condenser-evaporator, which is undesirable at this location for operational reasons. Within the context of the invention, the choice of heating means for the second condenser-evaporator can be selected as desired. In principle, any suitable process fraction can be used, for example, nitrogen, perhaps from the high-pressure column, any other fraction from the high-pressure column, a part-stream of the charge air or a fraction from a crude argon column which is connected to the low-pressure column, in particular crude argon from the top of a crude argon column of this type.
The xe2x80x9cpurge liquidxe2x80x9d of the first condenser-evaporator serves as a charge fraction for the krypton-xenon enrichment column. In the present context, the term xe2x80x9ckrypton-xenon enrichment columnxe2x80x9d is understood as meaning a countercurrent mass transfer column in which a fraction which has a higher concentration of krypton and/or xenon than each of the charge fractions of this column is produced. By way of example, the krypton-xenon concentrate has a higher molar level of krypton and/or xenon than the xe2x80x9cpurge liquidxe2x80x9d which is fed into the krypton-xenon enrichment column. This column may, for example, be designed as a transfer column, as described in DE 1000017 A1, and/or may at the same time be used to expel methane.
It is preferable for the purge liquid to be introduced in the lower region, for example directly above the bottom. In this case, a liquid is added to the top of the krypton-xenon enrichment column, in order to force the krypton which is present in the rising vapour downwards and to force methane upwards. This liquid may, for example, be removed from the high-pressure column, for example from the bottom of this column or a few plates above it. A possible alternative or additional source is the evaporation space of the top condenser of a pure argon column. In the bottom of the krypton-xenon enrichment column, the liquid flowing down can be boiled by means of a bottom evaporator. This allows the krypton and xenon contents of the krypton-xenon concentrate to be increased further. The bottom evaporator can be operated, for example, with compressed air or with compressed nitrogen from the top of the high-pressure column.
In the invention, an intermediate step, in the form of a partial evaporation in the first condenser-evaporator, may be carried out between the extraction of the krypton- and xenon-containing fraction from the high-pressure column and the feeding of this fraction into the krypton-xenon enrichment column. This step is used to concentrate krypton and/or xenon even before the krypton-xenon enrichment column is reached. As a further effect, all the other components with a lower volatility than oxygen are guided with the purge liquid out of the partial evaporation into the krypton-xenon enrichment column and are in this way kept away from other parts of the installation, in particular the low-pressure column.
The krypton-xenon concentrate which is produced in the krypton-xenon enrichment column has a krypton content of, for example, 600 to 5 000 ppm, preferably 1 200 to 4 000 ppm, a xenon content of, for example, 60 to 500 ppm, preferably 120 to 400 ppm. Otherwise, it consists mainly of oxygen and typically up to about 10 mol % of nitrogen.
The invention can particularly advantageously be implemented as part of an air fractionation plant with argon production in which an argon-containing fraction from the low-pressure column is introduced into a crude argon rectification stage. The crude argon rectification stage is used in particular for argon-oxygen separation and may be carried out in one or more columns (cf. for example EP 377117 B2 or EP 628777 B1). The cooling of the crude argon rectification stage which is in any case required is, in the context of the invention, effected by the krypton- and xenon-containing fraction, an argon-enriched vapour from the crude argon rectification coming into indirect heat exchange with the evaporating krypton- and xenon-containing fraction in the first condenser-evaporator. The partial evaporation as part of the krypton-xenon production therefore simultaneously serves to produce reflux and/or liquid product in the crude argon rectification stage.
In many cases, there is a liquid charge-air stream, for example in the internal compression of one or more products. The liquefied air is often split between high-pressure column and low-pressure column, for example by being introduced into a vessel which is arranged inside the high-pressure column and part of the liquid being removed again from this vessel and passed to the low-pressure column. Within the context of the invention, it is expedient if, instead, an oxygen-containing liquid is extracted from the high-pressure column and introduced into the low-pressure column, this oxygen-containing liquid originating from a second intermediate point, which is arranged above the first intermediate point at which the liquid charge air is introduced into the high-pressure column. This ensures that the krypton and xenon which are present in the liquid charge air flows towards the bottom of the high-pressure column and is not passed into the low-pressure column, where it would be lost to the krypton-xenon production. Moreover, other low-volatility impurities are kept .away from the main condenser. According to this aspect of the invention, the liquefied air (or an oxygen-containing liquid of similar composition) is formed by substantially krypton-and xenon-free reflux liquid of the high-pressure column.
This aspect of the invention can advantageously be applied to any process in which a fraction from the high-pressure column is fed to a krypton-xenon production stage. Its use is not limited to processes and apparatus with partial evaporation of the krypton-and xenon-containing fraction. The same applies to the corresponding further configurations.
It is preferable for there to be no mass transfer elements, i.e., plates or packing, arranged between the first intermediate point and the second intermediate point. As a result, the oxygen-containing liquid has substantially the same composition as the air, apart from the undesirable components which boil at a higher temperature than oxygen.
In the high-pressure column, there may be barrier plates, the krypton- and xenon-containing fraction being extracted below the barrier plates and an oxygen-enriched liquid being removed above the barrier plates. Therefore, the oxygen-enriched liquid contains significantly less krypton and xenon than the krypton-and xenon-containing fraction and may, for example, be passed directly into the low-pressure column and/or used to cool the top condenser of a pure argon column, without significant quantities of krypton and xenon being lost as a result. The number of barrier plates is, for example, one to nine, preferably two to six (theoretical plates).
In addition to the purge liquid, a gaseous stream can be extracted from the evaporation space of the first condenser-evaporator and likewise fed to the krypton-xenon enrichment column, for example at the same point as the purge liquid. As a result, the krypton which is still present in the evaporated part of the krypton-and xenon-containing fraction is also fed to the krypton-xenon production stage.
In the process, refrigeration can be generated by work-performing expansion of airxe2x80x94for example in a medium-pressure turbinexe2x80x94to approximately the operating pressure of the high-pressure column, which regularly involves partial liquefaction of the air. Within the context of the invention, this air which has been expanded in a work-performing manner can be fed to a phase separation, and at least part of the liquid fraction from the phase separation can be fed to the krypton-xenon enrichment column and/or the evaporation space of the first condenser-evaporator.
As an alternative or in addition, air may be expanded in a work-performing manner to approximately low-pressure column pressure, for example in a low-pressure turbine. The krypton and xenon which are present in the low-pressure air stream can be recovered if this air stream is fed to a stripping column and the bottom liquid from the stripping column is fed to the krypton-xenon enrichment column, preferably at the top or at an intermediate point a few plates below it. Moreover, the stripping column also retains other low-volatility components, such as N2O, which are undesirable in the low-pressure column.
The advantages of the aspects of the invention which are associated with the work-performing expansion of air are not restricted to processes and apparatus with partial evaporation of the krypton- and xenon-containing fraction. Rather, these process steps may also be used in other processes for krypton-xenon production.
In addition, the invention relates to an apparatus for producing krypton and/or xenon by low-temperature fractionation of air comprising:
a rectification system for nitrogen-oxygen separation comprising at least a high-pressure column (2), and a low-pressure column (3), and a charge-air line (1) for introducing compressed and precleaned charge air into the rectification system,
a first condenser-evaporator (17),and a removal line (13, 14, 15, 16, 416) for removing a krypton- and xenon-containing fraction from the high-pressure column (2),and introducing the krypton- and xenon-containing fraction into the evaporation space of the first condenser-evaporator (17),
a krypton-xenon enrichment column (24)and a purge-liquid line (26, 226) connected to the evaporation space of the condenser-evaporator (17) and to the krypton-xenon enrichment column (24), and
a product line (30) for removing a krypton-xenon concentrate from the krypton-xenon enrichment column (24), and
a second condenser-evaporator (27), separate from the first condenser-evaporator (17), wherein the evaporation space of the second condenser-evaporator (27) is in flow communication with the lower region of the krypton-xenon enrichment column (24).
According to an additional aspect of the invention, the apparatus further comprises an argon transfer line (48) connected to the low-pressure column (3) and connected to a crude argon rectification stage (18, 19), and the liquefaction space of the first condenser-evaporator (17) is in flow communication with the crude argon rectification stage (18, 19).
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German Application No. 101 53 252.0, filed Oct. 31, 2001, and of corresponding European Application No. 02001356.1, filed Jan. 18, 2002, is hereby incorporated by reference.